In computer science, a virtual machine (VM) is a portion of software that, when executed on appropriate hardware, creates an environment allowing the virtualization of an actual physical computer system. Each VM may function as a self-contained platform, running its own operating system (OS) and software applications (processes). Typically, a hypervisor manages allocation and virtualization of computer resources and performs context switching, as may be necessary, to cycle between various VMs.
A host machine (e.g., computer or server) is typically enabled to simultaneously run multiple VMs, where each VM may be used by a local or remote client. The host machine allocates a certain amount of the host's resources to each of the VMs. Each VM is then able to use the allocated resources to execute applications, including operating systems known as guest operating systems. The hypervisor virtualizes the underlying hardware of the host machine or emulates hardware devices, making the use of the VM, transparent to the guest OS or the remote client that uses the VM.
In a distributed virtualization environment, files associated with the VM, such as the OS, application, and data files, are all stored in a file or device that sits somewhere in shared storage that is accessible to many physical machines. Managing VMs requires synchronizing VM disk metadata changes between host machines to avoid data corruption. Such changes include creation and deletion of virtual disks, snapshots etc. The typical way to do this is to use either a centrally managed file system (e.g., Network File System (NFS)) or use a clustered file system (e.g., Virtual Machine File System (VMFS), Global File System 2 (GFS2)). Clustered file systems are very complex and have severe limitations on the number of nodes that can be part of the cluster (usually n<32), resulting in scalability issues. Centrally-managed file systems, on the other hand, usually provide lower performance and are considered less reliable.
Some virtualization systems utilize a Logical Volume Manager (LVM) to manage shared storage of VMs. An LVM can concatenate, stripe together, or otherwise combine shared physical storage partitions into larger virtual ones that administrators can re-size or move. Conventionally, an LVM used as part of a virtualization system would compose a VM of one or more virtual disks, where a virtual disk would be one or more logical volumes. Initially, a virtual disk would be just one logical volume, but as snapshots of the VM are taken, more logical volumes are associated with the VM. The use of an LVM in a virtualization system solves the scalability issue presented with a clustered file system solution, but still introduces administrative problems due to the complication of working directly with raw devices and lacks the ease of administration that can be found with use of a file system.